The world’s largest ultraviolet drinking water treatment facility, the $1.5 billion Catskill/Delaware UV Facility, provides treatment specifically for Cryptosporidium and Giardia microorganisms in the drinking water consumed by more than 9 million residents of the city and parts of Westchester County.

The 270,000-square-foot facility is designed to treat more than 2 billion gallons of water each day, more than three times the capacity of the next-largest municipal UV treatment facility in the United States, which is under construction in Los Angeles, according to DEP.

A 1993 Cryptosporidium outbreak in Milwaukee caused new federal regulations for treating drinking water to be created, because Cryptosporidium is resistant to disinfection with chlorine. Researchers then discovered that exposing water to low levels of UV light makes Cryptosporidium and Giardia harmless to human beings.

The Catskill/Delaware UV Facility 2006 began treating water in late 2012 as its construction continued; the construction is now substantially complete. The facility has 56 UV units and 11,760 UV bulbs overall.